The present invention relates to a method for pressing metal powder to form a consolidated body under utilization of sheet metal, and more particularly, the invention relates to shaping parts, having dimensions which are large in direction of powder pressing as compared with transverse dimensions such as a diameter or a wall thickness of hollow parts, etc.
The art of powder metallurgy is well developed and many parts, such as levers, bushings, bearing parts, etc., are made by filling an appropriate die with powder, and by completing the part through pressing. Subsequently, the part is heat treated (sintered) followed, in cases, by coining or sizing to improve accuracy in the dimensions if such improvement is indeed necessary.
It is also known to press powder into circular or annular blanks, and to forge or extrude these blanks into semi-finished products. In order to protect the blank from dirt and oxidation one usually clads or envelopes the blank into a sheet metal jacket.
Another method of working powder is known to involve powder of spherical particles which are placed into capsules and pre-compacted by means of ultrasonic vibrations to obtain a density of about 60 to 70% of the theoretically obtainable maximum density and to finally compress the particles by isostatic pressure (about 1500 to 5000 bars) to about 80 to 93% of that density. Only upon complying with these conditions, wrinkles will not be produced in the capsule during subsequent extrusion. Such wrinkles result in severe surface defects in the extruded section. The capsule is usually made of a highly ductile material such as nickel.
Isostating compression causes exertion of pressure upon the blank from all directions and in all dimensions. Thus, upon isostatically compressing a circular blank, one reduces its height as well as its diameter. Different powders are compressible to different degrees and differences in density during charging of the die render it rather difficult to obtain a shape whose geometry is sufficiently exact. Such accuracy is needed for the subsequent extrusion.
Other difficulties in powder working relate to particulars of the material. For example, some alloyed steels and other material whose composition is determined by particular purposes of subsequent use, upon being atomized by gas, may yield spherical particles. Such particles are difficult to compact into a state of sufficient coherency and consolidation.
It is further known generally to press metal powder into particular geometric shapes under utilization of steel dies. This widely used method is applicable to powder which flows rather easily into the die cavity and whose particles interlink and/or weld to each other upon being compressed in order to obtain the consolidation. Spherical particles usually flow adequately easily but they do not mechanically link up, and in many cases they weld only at very high pressures. These drawbacks are particularly noticeable when parts are to be made having a length greater than the diameter or greater than the wall's thickness in the case of a hollow. Particularly, one has not yet been able to produce satisfactorily parts under utilization of ball-shaped powder particles, in a steel die or in an isostatic press and without sheet jacket, which part is free from lubricant; geometrically accurate as to its contour; and sufficiently stabile so as to be suitable as a blank in a hot extruder. Spherical particles can be compressed in a hardened steel die and in the absence of a lubricant if the pressure is very high, but the edge strength is usually unsatisfactory for further use. Low pressure fails to compact the powder; the powder particles will flow out of the die upon opening the cavity.
Compressing powder consisting of spherically-shaped particles in a thin, ductile sheet metal jacket by means of an isostatic press, does not yield sufficiently accurate dimensions, needed for further working. Moreover, a thin nickel jacket is difficult to remove, particularly upon extruding. Moreover, a nickel jacket is expensive.
Compressing spherical powder in capsules by means of a plunger and a steel die did also fail to produce satisfactory results for press-forming parts having relatively long dimensions in the direction of punch movement, because the sheet metal capsules were strongly deformed and became wrinkled.